1. Field of the Invention
The invention generally relates to a flash light device, and more particularly, to a flash light device able to reduce the over-exposure problem.
2. Description of Related Art
In general speaking, both the digital camera and the film-based camera usually respectively have an embedded flash light to facilitate the user conducting light-complementing on the object to be shot in a dim light environment or a backlight environment. Since a flash light has a light intensity much higher than the environment's light intensity, so that when a flash light directly irradiates onto an object, if the flash light is too close to the object or the material of the object has easily-reflecting characteristic, the captured image is likely over-exposed.
FIG. 1 is a schematic flowchart of the flashing process of a conventional flash light. Referring to FIG. 1, in order to conduct light-complementing on an object to be shot by using a flash light, first, an original image without conducting light-complementing on the object to be shot by using the flash light is captured (step S102). Next, a pre-flash image is captured by using the flash light to conduct pre-flashing (step S104). Thereafter, the reflectivity of each pixel of the object to be shot is calculated according to the original image and the pre-flash image (step S106). Then, the flashing intensity of the flash light is determined according to a reflectivity of the object (step S108). Finally, the flash light is triggered for conducting light-complementing (step S110).
Although in step S104, for estimating the flashing intensity of the flash light required during shooting, a pre-flash method is used, but the flashing intensity of the flash light is not stable. Even though a same driving voltage and a same discharging time are given each time, the flashing intensity of the flash light is somehow different from time to time, as shown by FIG. 2 which illustrates the flashing intensities of the flash light during a test. In FIG. 2, all coordinate points are obtained by using the same driving voltage and the same discharging time for driving the flash light, but it is obvious the variation of the light energy of the flash light is quite large. Therefore, if light-complementing is conducted by using the same driving voltage to apply the flash light, it is possible to have two extreme situations of over-diming (the flashing intensity falls at, for example, point A) or over-exposing (the flashing intensity falls at, for example, point B). In this regard, how to effectively improve the light-complementing quality of a flash light has become an issue to be solved.
US Patent Application No. US2007/0121072 discloses a scheme, in which a liquid crystal lens (LC lens) is used to change the energy distribution of a flash light according to a distance between the object to be shoot and the LC lens. When the distance is short, i.e., the lens is close to the object to be shoot, the LC lens is driven to present a first distribution of indices of refraction so as to disperse the energy of the flash light, i.e., to avoid the above-mentioned over-exposing problem; on the contrary, when the object to be shoot is more far away from the lens, the LC lens is driven to present a second distribution of indices of refraction so as to concentrate the energy of the flash light, i.e., to concentrate the energy of the flash light on the object to be shoot. In this way, although the energy output of the flash light can be controlled to solve the over-exposing problem due to the distance between the object and the lens, however, it is unable to effectively reduce the over-exposing for an object with high reflectivity.